Institut Charles Sadron Event

Polymer ultrathin films are widely studied because of their endless technological and industrial applications. When the thickness is reduced, the surface-to-volume ratio increases altering various physical properties like the glass transition temperature or the coefficient of thermal expansion. The variation of mass density in such strong confinement is still highly controversial and debated as most of the assessments are made indirectly through model-dependent methods. We put forward in this work two new experimental approaches based on i) the adsorption of nanoparticles and ii) the dissolution of polymer films monitored by a quartz crystal microbalance (QCM) allowing for the first time to directly measure subtle changes in density as a function of film thickness. We have shown an unambiguous and significant variation in density when the thickness is reduced below a hundred nanometers for two common polymers: Polystyrene (PS) and Polymethylmethacrylate (PMMA). : an increase for PS and a decrease for PMMA films[1]. A result in full agreement with our previous work using nanoparticles adsorption[2] and with recent literature on the variation of the optical properties of these films[3-4] and magnetic levitation[5]. In a last part, we have shown that the density and coefficient of thermal expansion (CTE) of both PS and PMMA films present a similar behavior with thickness suggesting a possible correlation between Tg, CTE and free volume of polymer thin films.